Low dielectric constant amorphous fluorinated carbon and method of preparation

ABSTRACT

An amorphous fluorinated carbon film for use as a dielectric insulating layer in electrical devices is formed from a fluorinated cyclic hydrocarbon precursor. The precursor may be selected from the group consisting of hexafluorobenzene, 1,2-diethynyltetrafluorobenzene and 1,4-bis(trifluoromethyl) benzene. The film is deposited by a radiation or beam assisted deposition technique such as an ion beam assisted deposition method, a laser assisted deposition method, or a plasma assisted chemical vapor deposition method. The film is thermally stable in non-oxidizing environment at temperatures up to 400° C. and has a low dielectric constant of less than 3.0. The film can be suitably used as an insulator for spacing apart conductors in an interconnect structure.

This is a divisional of copending application(s) Ser. No. 08/608,893filed on Feb. 29, 1996, pending.

FIELD OF THE INVENTION

The present invention generally relates to amorphous fluorinated carbonfilms and method of preparation and more particularly, relates toamorphous fluorinated carbon films that are thermally stable and havelow dielectric constants suitable for use as insulating layers inelectronic devices and a method of preparation of such films.

BACKGROUND OF THE INVENTION

A semiconductor chip consists of an array of devices whose contacts areinterconnected by patterns of metal wiring. For instance, in VLSI chips,the metal patterns are multiayered and are separated by layers of aninsulating material which is characterized generally by a low dielectricconstant. Interlayer contacts between metal wiring patterns are made bythrough-holes which are etched through the layers of the insulatingmaterial. In a typical chip design, there are or more wiring layers. Lowdielectric constant insulating materials are employed between the wiresin the same level and also between the various wiring levels.

In a typical VLSI chip, the insulating material is silicon dioxide thathas a dielectric constant of between about 3.9 and about 4.1. As thespeed of the chip is affected by the RC value of the insulator, enhancedspeed performance requires reduction of the capacitance (C). The questfor higher integration in the chip results in the shrinkage of thedimension and tends to increase the capacitance values, unless thedielectric constant of the insulator is significantly reduced.Furthermore, with the increasing use of large scale integration in thechip design, back end wiring densities are increasing. As the wiringdensity increases, the need for lower dielectric constant insulatingmaterials, i.e. interlayer dielectric (ILD) materials arises in order toimprove the performance of the VLSI and ULSI devices.

Different materials that have low dielectric constants have beeninvestigated as potential replacement materials for silicon dioxide. Forinstance, among the candidate materials for the ILD, fluorinated carbonpolymers appear to have the lowest dielectric constant values, i.e., <3.However, most of the materials with significantly lower dielectricconstants such as those of fluoropolymers are thermally unstable at chipprocessing temperatures above 350° C., thus making them unsuitable forintegration in modem semiconductor fabrication technology. A thermalendurance at processing temperatures higher than 400° C. after thedeposition of the BEOL dielectric is frequently required in suchtechnology.

It is therefore an object of the present invention to provide a lowdielectric constant insulating material for use as a dielectricinsulating layer in electronic devices that does not have the drawbacksand shortcomings of the prior art dielectric materials.

It is another object of the present invention to provide a lowdielectric constant material for use as a dielectric insulating layer inelectronic devices that is thermally stable at semiconductor processingtemperatures.

It is a further object of the present invention to provide a lowdielectric constant insulating material for use as a dielectricinsulating layer in semiconductor devices that is thermally stable innon-oxidizing environment at temperatures up to 400° C.

It is another further object of the present invention to provide a lowdielectric constant insulating material for use as a dielectricinsulating material in both interlayer applications and intralayerapplications in a semiconductor device that is thermally stable innon-oxidizing environment.

It is still another object of the present invention to provide a lowdielectric constant insulating material for use as a dielectricinsulating layer in semiconductor devices that has a dielectric constantof less than 3.0.

It is another further object of the present invention to provide a lowdielectric constant for spacing apart one or more levels of conductorsin a semiconductor device.

It is still another farther object of the present invention to provide alow dielectric constant amorphous fluorinated carbon material for use asa dielectric insulating layer in semiconductor devices that can beformed from a fluorinated cyclic hydrocarbon precursor.

It is yet another further object of the present invention to provide alow dielectric constant amorphous fluorinated carbon material for use asa dielectric insulating layer in semiconductor devices that can beformed from a fluorinated cyclic hydrocarbon precursor such ashexafluorobenzene, 1,2-diethynyltetrafluorobenzene or1,4-bis(trifluoromethyl) benzene.

It is yet another further object of the present invention to provide alow dielectric constant amorphous fluorinated carbon material for use asa dielectric insulating layer in VLSI or ULSI devices that can bedeposited by a radiation or beam assisted deposition technique.

SUMMARY OF THE INVENTION

In accordance with the present invention, an amorphous fluorinatedcarbon film for use as a dielectric insulating layer in semiconductordevices formed from a fluorinated cyclic hydrocarbon precursor and amethod of preparation are provided.

In the preferred embodiment, the amorphous fluorinated carbon film foruse as a dielectric insulating layer is formed from a precursor selectedfrom the group consisting of hexafluorobenzene,1,2-diethynyltetrafluorobenzene and 1,4-bis(trifluoromethyl) benzene.The film is prepared by a radiation or beam assisted depositiontechnique such as an ion beam assisted method, a laser assisteddeposition method or a plasma assisted chemical vapor deposition method.The film deposited is thermally stable in non-oxidizing environment attemperatures up to 400° C. and has a dielectric constant of less than3.0. The film can be used as an interconnect dielectric layer in anelectronic device, for instance, as an insulator for spacing apartconductors in an interconnect structure.

In an alternate embodiment, the amorphous fluorinated carbon film isdeposited on an intermediate non-fluorinated diamond-like carbon layeror between two non-fluorinated diamond-like carbon layers.

In another alternate embodiment, an additional graded layer is depositedbetween the fluorinated carbon layer and the non-fluorinateddiamond-like carbon layer.

The present invention is also directed to an insulator used for spacingapart one or more levels of conductors in a semiconductor device whichincludes a substrate that has a top surface with an exposed first layerof metal, an insulator layer of amorphous fluorinated carbon formed froma fluorinated cyclic hydrocarbon precursor, and a second layer of metalthat is patterned to form a plurality of conductors on the insulatorlayer. The substrate may also have an upper surface with an exposedfirst area of metal and an exposed second area of an insulatingmaterial. The second area of insulating material is used for intra-levelinsulation purpose. The insulator may further include a graded layerbetween the fluorinated carbon layer and the non-fluorinateddiamond-like carbon layer to improve the adhesion between the twodielectric layers. A continuous transition is provided by the gradedlayer between the two dielectric layers.

The present invention is further directed to an interconnect structurefor use in a semiconductor device that includes a substrate, a firstlayer of an electrically conductive material deposited on the surface ofthe substrate, a layer of an amorphous fluorinated carbon deposited ontop of the first layer of electrically conductive material from afluorinated cyclic hydrocarbon precursor, a second layer of anelectrically conductive material deposited on the amorphous fluorinatedcarbon layer, and a metal stud connecting the first layer ofelectrically conductive material to the second layer of electricallyconductive material. The first and the second electrically conductivematerial can be aluminum, copper, tungsten, tantalum, titanium, theiralloys and conductive metal nitrides. The layer of the amorphousfluorinated carbon has a low dielectric constant of smaller than 3.0 andsome layers have a dielectric constant smaller than 2.8. It is thermallystable in a non-oxidizing environment at temperatures up to 400° C. Inone alternate embodiment, the layer of the fluorinated carbon isdeposited on an intermediate non-fluorinated diamond-like carbon layeror between two non-fluorinated diamond-like carbon layers. In anotheralternate embodiment, the interconnect structure also include a gradedlayer between the fluorinated carbon layer and the non-fluorinateddiamond-like carbon layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Other object, features, and advantages of the present invention willbecome apparent upon consideration of the specification and the appendeddrawings in which:

FIG. 1 is an enlarged cross-sectional view of a semiconductor devicewhich contains an amorphous fluorinated carbon layer as in insulatinglayer for spacing apart two levels of metalization in a semiconductordevice.

FIG. 2 is an enlarged cross-sectional view of a Field Effect Transistorof a CMOS structure which contains amorphous fluorinated carbon films asa first and a second dielectric insulating layer.

FIG. 3 is an enlarged cross-sectional view of an ULSI interconnectstructure of the present invention which contains amorphous fluorinatedcarbon films used as the interlevel and the intralevel dielectric layer.

DETAILED DESCRIPTION OF THE PREFERRED AND THE ALTERNATE EMBODIMENTS

The present invention provides an amorphous fluorinated carbon film foruse as a dielectric insulating layer in electronic devices formed from afluorinated cyclic hydrocarbon precursor and a method for suchpreparation.

The present invention discloses a method of preparation of novel lowdielectric constant materials of high thermal stability that aresuitable as dielectrics in VLSI and ULSI interconnects. According to theinvention, thermally stable materials of amorphous fluorinated carbon(a-F:C) films are prepared from fluorinated cyclic hydrocarbons usingradiation or beam assisted techniques such as ion beam assisteddeposition, laser assisted deposition or plasma assisted chemical vapordeposition (PACVD). The more preferred method is PACVD which allows theuniform deposition of the low-epsilon material over a large area.Typical precursor materials for the preparation of the amorphousfluorinated carbon films are hexafluorobenzene (C₆ F₆),1,2-diethynyltetrafluorobenzene (C₆ F₄ (C₂ H)₂), and1,4-bis(trifluoromethyl) benzene (C₆ H₄ (CF₃)₂).

The films and their method of preparation are suitable for integrationin the semiconductor fabrication technology. The films can be depositeduniformly over large areas by radio frequency or DC PACVD, preferably onnegatively biased substrates. The substrate temperature can be set atbetween room temperature and 250° C. The crosslinked films thus preparedhave low dielectric constants, have no structural anisotropy, and arecharacterized by high electrical resistivity and chemical inertness.

In a typical PACVD process, a semiconductor device is first loaded intoa parallel plate plasma reactor and electrically connected to become oneof the electrodes. After the reactor is pumped to a preset negativepressure, a suitable mixture of reactant gases of fluorinated cyclichydrocarbon vapors and hydrogen is flown into the reactor, while thepressure inside the reactor is maintained at a preset value in the rangebetween about 30 m Torr and about 300 m Torr. A DC or RF power is thenapplied to the electrodes of the reactor to ignite a plasma such thatthe device to be coated becomes negatively biased relative to ground orto other parts of the reactor. The device is kept in the plasma untilthe required thickness of coating is obtained. A desirable coatingthickness is in the range between about 0.3 μm and about 1.5 μm perlayer.

EXAMPLE 1

One example of depositing a-F:C films on an eight inch wafer is to use aparallel plate RF PACVD system and hexafluorobenzene as the precursorunder the following chamber conditions:

Flow Rate=10 sccm

Pressure=30 m Torr

Substrate bias=-100 V D C

RF power=25 Substrate

temperature=180° C.

An amorphous fluorinated carbon film of 1 μm thickness is obtained aftera deposition time of 20 minutes.

EXAMPLE 2

Another example of depositing a-F:C films on an eight inch wafer is touse a parallel plate DC PACVD system and hexafluorobenzene as theprecursor under the following chamber conditions:

Flow Rate=20 sccm

Pressure=100 m Torr

Substrate bias=-500 V D C or -800 V D C

DC Power density=0.1 W CM⁻²

Substrate Temperature=250° C.

An amorphous fluorinated carbon film of 1 μm thickness is obtained aftera deposition time of 20 minutes.

The films deposited have a dielectric constant of ≦2.8 and are thermallystable in vacuum or helium for at least four hours at 400° C. The filmscontain fluorine in a concentration range between about 22 and about 42atomic percent. The amorphous fluorinated carbon film can be patternedfor vias and/or planarized by using an oxygen reactive ion etchingtechnique. The films are therefore suitable for use as isotropic lowdielectric constant insulators in integrated electronic devices orpackaging, for instance, as back end structures for contact to CMOSdevices. The present invention novel amorphous fluorinated carbon filmscan also be utilized in an interconnect structure as the insulatinglayers. The interconnect structure normally includes a substrate thathas an upper surface with an exposed first area of a metal and anexposed second area of an insulating material, a first layer ofamorphous fluorinated carbon material formed on the upper surface of thesubstrate, a second layer of metal patterned to form a plurality ofconductors on top of the fluorinated carbon layer, and a metal studelectrically connecting selected first areas to one or more of theplurality of conductors.

An integrated semiconductor device as described above is shown inFIG. 1. The integrated semiconductor device 10 includes a substrate 12which has an upper surface 14 including an exposed first area of metal16 and an exposed second area of insulating material 18. A layer offluorinated carbon material 20 is deposited on the upper surface of thesubstrate 14. A metal stud 24 is formed through the fluorinated carbonlayer 20 to connect the exposed first area 16 to a second metal layer 22which is subsequently formed. Suitable metals that can be used in thepresent invention interconnect structure include Al, Cu, W, Ti, Ta,alloys thereof and conductive metal nitrides. The metal can be depositedusing a sputtering or a chemical vapor deposition technique.

In the amorphous fluorinated carbon film, it may be desirable to containa small amount of hydrogen in the concentration range between about 0and about 20 atomic percent, and preferably between about 0 and about 5atomic percent. It has been discovered that such a small amount ofhydrogen improves the strength of the films. The total content ofhydrogen should be limited, however, since the dielectric constant ofthe film generally increases with the hydrogen content.

A second example of application of the present invention amorphousfluorinated carbon film is shown in FIG. 2 in a field effect transistoror FET. The FET contains an insulator for spacing apart one or morelevels of conductors in an integrated circuit chip. FIG. 3 shows a FETdevice 30 which includes a substrate 32 that has implanted source anddrain regions 34, a polycrystalline silicon gate region 36 and recessedoxide regions 38. The FET device 30 further includes a CVD W trenchfills 40, a passivating layer of silicon dioxide or silicon nitride 41,copper, aluminum or tungsten interconnects 42 and 43, a metal liner ofTi, Ta, W or compounds or alloys thereof 44, an interlevel via filledwith W, Al or Cu 45, a fluorinated carbon layer 46, and a capping layerof fluorinated carbon material, silicon dioxide, silicon nitride, boronnitride or compound thereof 47. The fluorinated carbon layer 46 of theFET device 30 acts as an insulator for spacing apart contact levels 42and 43 of the FET device. The fluorinated carbon insulator has a lowdielectric constant which is uniform in all directions. The fluorinatedcarbon insulator taught by the present invention therefore presents asignificant improvement over prior art insulators.

A third example of application of the present invention amorphousfluorinated carbon film is illustrated in FIG. 3 in an ULSI interconnectstructure. The ULSI interconnect structure 50 includes a substrate 52,Cu interconnects and studs 54, a Ta liner 56, fluorinated carboninterlevel and intralevel dielectric layers 58, a Si-doped fluorinatedcarbon or Si-doped non-fluorinated diamond-like carbon RIE stop, and abarrier layer 60. It should be noted that the layer of amorphousfluorinated carbon film can be formed on an intermediate non-fluorinateddiamond-like carbon layer, or formed between two non-fluorinateddiamond-like carbon layers. The interconnected structure may furtherinclude a graded layer between the fluorinated carbon layer and thenon-fluorinated diamond-like carbon layer. The use of the graded layeris to improve the adhesion between the two dielectric layers byproviding a continuous transition between the two layers.

While the present invention has been described in an illustrativemanner, it should be understood that the terminology used is intended tobe in a nature of words of description rather than of limitation.

Furthermore, while the present invention has been described in terms ofa preferred and several alternate embodiments thereof, it is to beappreciated that those skilled in the art will readily apply theseteachings to other possible variations of the invention. For instance,other layered structures of the fluorinated carbon films and thenon-fluorinated diamond-like films may be used and farther, othercoating methods may be employed to deposit the films while substantiallyachieve the same desirable results of the present inventions.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method of forming anamorphous fluorinated carbon film by a radiation or beam assisted methodcomprising the step of flowing into a reaction chamber a fluorinatedcyclic hydrocarbon precursor and reacting said precursor to form saidfilm comprising fluorine in a concentration range between about 22 andabout 42 atomic percent.
 2. A method according to claim 1, wherein saidfluorinated cyclic hydrocarbon precursor is selected from the groupconsisting of hexafluorobenzene, 1,2-diethynyltetrafluorobenzene and1,4-bis(trifluoromethyl) benzene.
 3. A method according to claim 1,wherein said radiation or beam assisted method is selected from thegroup consisting of an ion beam assisted deposition technique, a laserassisted deposition technique and a plasma assisted chemical vapordeposition technique.
 4. A method according to claim 1, wherein saidamorphous fluorinated carbon film is used as an insulator for spacingapart conductors in an interconnect structure.
 5. A method according toclaim 1, wherein said fluorinated carbon film is deposited on anintermediate non-fluorinated diamond-like carbon layer.
 6. A methodaccording to claim 1, wherein said fluorinated carbon film is depositedbetween two non-fluorinated diamond-like carbon layers.
 7. A methodaccording to claim 5 further comprising the step of depositing a gradedlayer between said fluorinated carbon layer and said non-fluorinateddiamond-like carbon layer.
 8. A method according to claim 1, whereinsaid fluorinated carbon film deposited has a dielectric constant smallerthan 3.0.